You cannot map the shifter gears in GT5. Your Hi Converter sequential mod will only work on games where you can re-map the shifter gears 3 and 4 to "shift up/down." More common to map this in PC games than PS3 games.
There is a modification for the G27 to make the G25 shifter work on it. I am not totally sure if you can use full sequential with it, but if I remember correctly that was the result of it.
Or just build a Sequential shifter off your ds3 similar to how you would do a handbrake mod. See my GTp album for pics on mine I used a flight sim stick linked to ds3's R3 stick.